In recent years, along with a trend for high quality of glasses for vehicles, an optimum glass is required for each application site. For example, for a rear window glass of an automobile, a glass which has, in a thickness of 5 mm, a visible light transmittance (illuminant A) of from 25 to 40% and a solar radiation transmittance of from 10 to 30% is used, and for a sunroof, a glass which has a visible light transmittance (illuminant A) of at most 30% and a solar radiation transmittance of at most 20%, is frequently used. The properties commonly required for these glasses are such that they have low visible light transmittance and low solar radiation transmittance, and preferably low ultraviolet ray transmittance, particularly low light transmittance at a wavelength of 370 nm, is desired.
Meantime, in recent years, a green colored glass which has high visible light transmittance and which at the same time has low solar radiation transmittance and low ultraviolet ray transmittance, has been used for a windshield, and to secure harmony in the color of the vehicle, it is preferred that the rear window glass or the like also has a green type color.
Most of well known heat absorbing glasses having relatively low visible light transmittance contain nickel. However, nickel is not desirable, since it sometimes forms nickel sulfide in glass. Nickel sulfide is almost indistinguishable by naked eyes and presents no adverse effects to glass in a usual state. However, if nickel sulfide is present in glass after toughening treatment by quenching, the volume will expand due to crystal transformation at room temperature, and a thermal stress sufficient to break the glass may sometimes be formed.
JP-A-2-64038 (which corresponds to U.S. Pat. No. 4,873,206) or JP-A-4-275943 (which corresponds to EP-A-482253) discloses a glass which contains no nickel and which has a visible light transmittance of lower than 40%. However, the one disclosed in JP-A-2-64038 has a problem that the solar radiation transmittance tends to be high, since its infrared ray shielding ability is not so high. The one disclosed in JP-A-4-275943 has a large content of iron and thus has a practical problem in its production such that it takes time to change the base material.
Further, JP-A-6-247740 discloses a glass having visible light transmittance reduced by replacing a NiO component by a chromium component. However, this relates to a glass having a relatively long dominant wavelength and low excitation purity by an addition of selenium in an amount of at least 15 ppm, and it is intended to obtain a neutral (gray) color i.e. not a green color.
Still further, U.S. Pat. No. 5,411,922 discloses a glass having low visible light transmittance which contains at least 10 ppm of selenium as an essential component in addition to iron, cobalt and titanium. In a glass melting furnace, Se is most volatile and an expensive material, and among Se put into the furnace, at least 90% will be discharged out of the furnace together with a combustion exhaust gas and will be captured, for example, by an absorbing liquid in an absorption tower for flue gas desulfurization. The major portion of Se in the liquid will be recovered and utilized as a raw material, but in order to carry out treatment to sufficiently reduce Se ions remaining in the liquid after the recovery, large amounts of investment for installation and running costs will be required. In Japan, a regulation relating to Se concentration in an industrial waste water is expected to come into force from 1997.
Further, Examples 11 to 13 in U.S. Pat. No. 5,411,922 describe an influence of an addition of titanium in a certain specific glass containing iron, cobalt and selenium as essential components. According to the description, as the titanium content is increased from 0 to 0.63 wt %, the dominant wavelength changes from 484.6 nm to 541.4 nm, and at the same time, the excitation purity decreases from 9.3% to 1.8%, whereby a neutral (gray) color tends to be readily obtainable.
Japanese PCT publication JP-6-503300 (which corresponds to EP-A-536049) discloses that to obtain a heat absorbing glass having a visible light transmittance of less than 40%, iron is added in an amount of at least 1 wt %, or Cr.sub.2 O.sub.3 is added in an amount of at least 0.025 wt %. The same is disclosed in French Patent 2,682,102 which corresponds to the basic application from which the convention priority is claimed in EP-A-536049.
As mentioned above, addition of iron in an amount of at least 1 wt % may bring about a practical problem such that it takes time to change the base material. On the other hand, addition of Cr.sub.2 O.sub.3 in a substantial amount is likely to lead to formation of a foreign substance due to formation of a chromite. Accordingly, the technique based on a combination of iron and chromium only as disclosed in Japanese PCT publication JP-6-503300 is hardly applicable to a case where a glass having still lower visible light transmittance is to be obtained. Further, even in a case where a glass having a visible light transmittance at a level of from 25 to 40% is to be obtained, it is a substantial merit that a lower iron content and a lower chromium content can both be satisfied from a viewpoint such that stable production can thereby be made possible.
Further, EP-A-653388 discloses a gray glass having at least 50 ppm of NiO and/or at least 9 ppm of Se added.
An object of the present invention is to solve the above problems of the prior art and to obtain a glass which contains no or little Se and has relatively low contents of total iron and chromium in spite of low visible light transmittance and solar radiation transmittance and which exhibits a dark green color.
Another object is to obtain a glass which is readily meltable and can be produced by a conventional float glass production process and which exhibits a dark green color with the ultraviolet ray transmittance controlled to be substantially low.